1. Field of the Invention
The present invention relates generally to an assisted global positioning system (AGPS), and in particular, to a method for creating GPS acquisition assistance information (hereafter referred to as “AA data”) required in terminal location determination in the AGPS.
2. Description of the Related Art
Location determination techniques using GPS satellites have come into wide use in various fields, particularly in location measurement of car and ship navigation systems and mobile communication terminals. To measure locations using the GPS satellites, a GPS receiver is mounted in a target device whose location is to be measured, such as in the car and ship navigation systems and mobile communication terminals, and the GPS receiver continuously receives GPS signals from the GPS satellites. However, such an operation consumes much power. In particular, since mobile terminals use batteries having a limited amount of power, the amount of time during which conversations can take place is reduced due to power consumption of the GPS receiver.
To solve such a problem, it has been suggested that a terminal should enable its GPS receiver only when location measurement is necessary. Although this method reduces power consumption of the battery of the terminal, it takes a long time for the GPS receiver to acquire GPS satellite signals required for location measurement.
Thus, to solve problems concerning power consumption of a GPS receiver and the amount of time taken by GPS satellite signal acquisition in a terminal, an AGPS has been used. The AGPS provides AA data for GPS signal acquisition to a terminal, thereby allowing the GPS receiver to more quickly acquire GPS signals using the provided AA data.
Hereinafter, a procedure for determining a location of a terminal in a general AGPS will be described with reference to FIG. 1, which is a block diagram of a general AGPS, and to FIG. 2, which is a flow diagram illustrating a terminal location determination procedure in the general AGPS.
Referring to FIG. 1, the AGPS includes a mobile communication terminal 10, a GPS satellite 20, a mobile communication network 30, a location determination server 40, and a reference station GPS receiver 50.
The mobile communication terminal (hereinafter, referred to as a “terminal”) 10 has a built-in GPS receiver, receives a GPS signal from the GPS satellite 20, and is connected to the location determination server 40 through the mobile communication network 30. In the case of a synchronous code division multiple access (CDMA) system, the mobile communication network 30 may comprise base stations 32, a base station controller 34, and a mobile switching center 36. By performing radio communication with the base stations 32, the terminal 10 is connected to the location determination server 40 via the base station controller 34 and the mobile switching center 36. In the case of an asynchronous wideband code division multiple access (WCDMA) system, the mobile communication network 30 may comprise a Node-B and a radio network controller (RNC), and the terminal 10 may perform radio communication with the Node-B and be connected to the location determination server 40 via the RNC. FIG. 1 shows the synchronous CDMA system.
The location determination server 40 has mounted thereon the reference station GPS receiver 50 and receives GPS satellite information through the reference station GPS receiver 50.
Referring to FIG. 2, the terminal 10 sends a terminal location determination request to the location determination server 40 in step 102. In some cases, the terminal location determination request can be generated in the location determination server 40. After sending the terminal location determination request, the terminal 10 sends an AA data request for GPS satellite signal acquisition to the location determination server 40 via the mobile communication network 30 in step 104. The GPS satellite signal is acquired by modulating navigation data at 50 Hz with an inherent pseudo noise code (also called a Gold code) for each satellite using spread spectrum, loading the spread navigation data on a carrier wave signal of about 1.5 GHz, and then modulating the resultant signal using binary phase shift keying (BPSK). Thus, in order for the terminal 10 to acquire the GPS satellite signal, the inherent pseudo noise code for each satellite and the carrier wave signal of 1.5 GHz must be removed from the GPS signal received by the GPS receiver. Information needed by the terminal for such removal 10 is called AA data. The AA data includes a GPS PRN (Pseudo-Random Number) that is observable by the terminal 10, a time of application (TOA), pseudo distance information and pseudo distance search range information of the GPS satellite 20, and Doppler frequency information and Doppler search range information of the GPS satellite 20.
The location determination server 40 creates the AA data using GPS satellite information in step 106 in response to the AA data request from the terminal 10. In other words, the location determination server 40 estimates an initial presumed location of the terminal 10 using base station information and determines the TOA. Then the location determination server 40 calculates pseudo distance information SV_CODE_PH of the GPS satellite 20 which can be received by the terminal 10 in the initial presumed location at the TOA, sets a pseudo distance search range SV_CODE_PH_WIN, calculates Doppler information DOPPLERO of the GPS satellite 20, and sets a Doppler search range DOPPLER_WIN. Also, the location determination server 40 creates the AA data using the calculated SV_CODE_PH, SV_CODE_PH_WIN, DOPPLERO, and DOPPLER_WIN.
The location determination server 40 provides the created AA data to the terminal 10 in step 108. In step 110, the terminal 10 acquires the GPS satellite signal using the AA data and measures the pseudo distance information of the GPS satellite 20. The method by which the terminal 10 acquires the GPS satellite signal using the AA data will be described with reference to FIG. 3.
After measuring the pseudo distance information of the GPS satellite 20, the terminal 10 provides the measured pseudo distance information of the GPS satellite 20 to the location determination server 40 in step 112. In step 114, the location determination server 40 calculates a location of the terminal 10 using the pseudo distance information of the GPS satellite 20 which is provided from the terminal 10. Then the location determination server 40 provides the calculated location information of the terminal 10 to the terminal 10 in step 116. Calculation of the location of the terminal 10, in step 114, may be undertaken by the location determination server 40, or if the location determination server 40 provides satellite orbit information (“Ephemeris”), the calculation can be performed by the terminal 10.
However, in the above location determination procedure on the terminal 10, if there is a repeater in the base stations 32, a repeater delay occurs. As a result, the actual SV_CODE_PH and DOPPLERO of the terminal 10 fall outside SV_CODE_PH_WIN and DOPPLER_WIN of the AA data. If the actual SV_CODE_PH and DOPPLERO of the terminal 10 fall outside SV_CODE_PH_WIN and DOPPLER_WIN of the AA data, the terminal 10 cannot acquire the GPS satellite signal.
FIG. 3 shows the pseudo distance information SV_CODE_PH, the pseudo distance search range SV_CODE_PH_WIN, the Doppler information DOPPLERO, and the Doppler search range DOPPLER_WIN, all of which are included in the AA data according to prior art. A procedure in which the terminal 10 acquires the GPS satellite signal using the AA data provided from the location determination server 40 will be described with reference to FIG. 3.
Referring to FIG. 3, the terminal 10 searches SV_CODE_PH_WIN around SV_CODE_PH within an entire code search range for pseudo distance information and searches DOPPLER_WIN around DOPPLERO within an entire Doppler search range for Doppler information, based on the AA data provided from the location determination server 40. In other words, since the terminal 10 searches a code and Doppler search range 160 corresponding to the AA data without searching an entire code and Doppler search range, searching can be performed quickly. If actual code and Doppler information of the terminal 10 falls within a predetermined range 150 of the code and Doppler search range 160 corresponding to the AA data, the terminal 10 can acquire the GPS satellite signal.
However, if the actual code and Doppler information of the terminal 10 falls within a range 155 that is outside the code and Doppler search range 160 corresponding to the AA data due to a repeater delay, the terminal 10 cannot acquire the GPS satellite signal. If the terminal 10 cannot acquire the GPS satellite signal, it is impossible to calculate the location of the terminal 10.
FIGS. 4A and 4B show pseudo distance information and a pseudo distance search range according to the prior art, where the actual code and Doppler information of the terminal 10 falls outside the code and Doppler search range 160 corresponding to the AA data.
In FIGS. 4A and 4B, SV_CODE_PH represents pseudo distance information calculated by the location determination server 40, SV_CODE_PH_WIN represents a pseudo distance search range that is set around the calculated SV_CODE_PH, and DELAY represents a location of a pseudo distance of the GPS satellite 20, which has been measured by the terminal 10 and is delayed by a repeater delay introduced by a repeater that exists in the base station 32. As shown in FIG. 4A, if the pseudo distance of the GPS satellite 20 measured by the terminal 10 falls outside the set SV_CODE_PH_WIN, the terminal 10 fails to acquire the GPS satellite signal and cannot acquire the location thereof.
Conventionally, to solve such a problem, the terminal 10 shifts the TOA of the AA data for GPS signal acquisition, or sets SV_CODE_PH_WIN and DOPPLER_WIN to cover a larger range.
FIG. 4B shows an example where the terminal 10 shifts the TOA of the AA data for GPS signal acquisition. In FIG. 4B, SV_CODE_PH shifted to the right side and SV_CODE_PH_WIN set around the shifted SV_CODE_PH are shown. In this way, if SV_CODE_PH is shifted and SV_CODE_PH_WIN is set around the shifted SV_CODE_PH, a time to first fix (hereinafter, referred to as a “TTFF”) that is required until the terminal 10 acquires the GPS satellite signal is increased by the amount of shift of SV_CODE_PH. Also, conventionally, the TOA is shifted without consideration of a bias error of a terminal clock, which is caused by a repeater delay of a serving base station. As a result, the terminal 10 often cannot acquire the GPS satellite signal even when the TOA is shifted.
Also, when SV_CODE_PH_WIN and DOPPLER_WIN are set to cover a larger range, the TTFF is increased and often the terminal 10 still cannot acquire the GPS satellite signal.
In conclusion, according to the prior art, if a bias error is included in an error of a terminal clock in an AGPS due to the occurrence of a repeater delay, there occurs large differences among the GPS satellite signal received by the terminal 10 and SV_CODE_PH and DOPPLERO of the AA data at the TOA. Thus, when the terminal 10 applies the AA data for GPS satellite signal acquisition, the GPS satellite signal received by the terminal 10 falls outside SV_CODE_PH_WIN and DOPPLER_WIN, resulting in failure to acquire a GPS satellite signal. As a result, the location of the terminal 10 cannot be measured.